We have known for several decades that the universe is expanding. Scientists use multiple techniques to measure the present-day expansion rate of the universe, known as the Hubble constant. These methods are internally consistent and based on the same physics, so all observed values of the Hubble constant should agree. But those that come from early-universe datasets disagree with those that come from late-universe datasets. This problem is known as the Hubble tension and is considered to be one of the most significant open questions in cosmology.
Now a team of astrophysicists, cosmologists, and physicists at The Grainger College of Engineering at the University of Illinois Urbana-Champaign and at the University of Chicago has developed a novel way to compute the Hubble constant using gravitational waves—tiny ripples in the spacetime fabric. The researchers were able to improve upon the accuracy of prior gravitational-wave methods of measuring the Hubble constant. As our capability to observe gravitational waves improves in the future, this new method can be used to make even more accurate measurements of the Hubble constant, bringing scientists closer to resolving the Hubble tension.
Illinois Physics Professor Nicolás Yunes said, “This result is very significant—it’s important to obtain an independent measurement of the Hubble constant to resolve the current Hubble tension. Our method is an innovative way to enhance the accuracy of Hubble constant inferences using gravitational waves.” Yunes is the founding director of the Illinois Center for Advanced Studies of the Universe (ICASU) on the Urbana campus.
UChicago Professor of Physics and of Astronomy & Astrophysics and co-author on the new research Daniel Holz comments, “It’s not every day that you come up with an entirely new tool for cosmology. We show that by using the background gravitational-wave hum from merging black holes in distant galaxies, we can learn about the age and composition of the universe. This is an exciting and completely new direction, and we look forward to applying our methods to future datasets to help constrain the Hubble constant, as well as other key cosmological quantities.”
In addition to Yunes and Holz, the team comprises Illinois physics graduate student Bryce Cousins, an NSF Graduate Research Fellow and lead author on the new study; Illinois physics graduate student Kristen Schumacher, an NSF Graduate Research Fellow; Illinois physics postdoctoral research associate Ka-wai Adrian Chung; and University of Chicago postdoctoral researchers Colm Talbot and Thomas Callister, both Kavli Institute for Cosmological Physics Postdoctoral Fellows. The paper is accepted for publication in Physical Review Letters and will appear in the March 11 issue. The full article is available now in the arXiv repository.
Previous measurements of the Hubble constant
Since the early 20th century, the various methods that have been used to measure the expansion rate of the universe have fallen into two major approaches, techniques using electromagnetic observations and those using gravitational-wave data. In the standard candle method, scientists leverage known qualities of supernovae—bright explosions of dying stars: both the distance of these events from Earth and the speed at which they are moving away can be easily computed. Scientists can use these two pieces of information to measure the expansion rate of the universe.
More recently, the detection of gravitational waves has enabled new methods for measuring the Hubble constant. Gravitational waves are generated by the energetic collisions of compact astrophysical objects such as black holes. These spacetime ripples are analogous to the concentric waves that spread out across the surface of a pond after a stone is thrown into the water. Gravitational waves travel at the speed of light and can eventually reach detectors on Earth. The global network that detects gravitational waves is operated by the LIGO-Virgo-KAGRA (LVK) Collaboration, which has over 2,000 members.
Using gravitational waves to calculate the Hubble constant is similar to the electromagnetic technique using supernovae, and the distances to black hole collisions can be easily determined using the standard siren method. However, the recessional velocity—the speed at which the collision point is moving away from Earth due to the expansion of the universe—cannot be determined directly. In order to compute the Hubble constant by this technique, astronomers must either identify light emitted by the merger or find its host galaxy.
Ideally these different techniques would yield consistent values of the Hubble constant, but this is not the case. If the Hubble tension cannot be resolved, it may be telling us something new about the early universe. All the various possible solutions to the Hubble tension include modifying the theory of the early universe’s energy composition or behavior, which would explain the differences in measured expansion rates. These proposed solutions include the presence of early dark energy, a proposed force that causes the universe to expand at an accelerating rate; or interactions between dark matter, a form of matter invisible to the eye that makes up most of the matter in the universe, and neutrinos, elementary particles that interact via gravity; or evolving dark-energy dynamics.
A new gravitational-wave method to measure the Hubble constant
In the current research, Yunes, Cousins, and collaborators propose an innovative method to measure the Hubble constant that leverages astrophysical collisions that the LVK network is not yet sensitive enough to detect, known as the gravitational-wave background.
“Because we are observing individual black hole collisions, we can determine the rates of those collisions happening across the universe. Based on those rates, we expect there to be a lot more events that we can’t observe, which is called the gravitational-wave background,” explains Cousins.
The team demonstrates that at lower values of the Hubble constant, there is a smaller total volume of space within which the collisions occur, so the density of object collisions is higher. This would increase the strength of the gravitational-wave background signal, so a lack of detection of the background thus excludes a lower value of the Hubble constant.
The researchers named this new method the stochastic siren method, noting that the collisions comprising the gravitational-wave background occur stochastically.
The team applied their stochastic siren method to current LVK Collaboration data. As a proof of principle, they found that the non-detection of the gravitational-wave background was able to provide evidence against slow universe expansion rates. They then combined the stochastic siren method with measurements of the Hubble constant from individual black hole collisions to produce a more accurate measurement of the expansion rate. This demonstrates that the team’s new gravitational-wave method is viable and shifts the measured value of the Hubble constant into the Hubble tension region.
As gravitational-wave detector sensitivity increases, the team’s method can be used to further improve measurements of the Hubble constant. The gravitational-wave background is expected to be detected within the next six years. Until that point, the stochastic siren method would constrain incrementally higher values of the Hubble constant as the upper limits on the background improve, providing another probe of the Hubble tension even without a full detection.
“This should pave the way for applying this method in the future as we continue to increase the sensitivity, better constrain the gravitational-wave background, and maybe even detect it,” says Cousins. “By including that information, we expect to get better cosmological results and be closer to resolving the Hubble tension.”
